The biological significance of double-stranded RNA (dsRNA) is reflected in the growing body of evidence which indicates that dsRNA exerts a potent inhibitory effect on a broad spectrum of eukaryotic biological systems, including animals, normal tissues, solid tumors, cultured cells, and cell-free extracts. Specific effects induced by dsRNA include: (a) the induction of interferon; (b) a cytotoxic effect on cultured cells; (c) regression of solid tumors and suppression of tumor transplantation; (d) enhancement of the immune response; and (e) inhibition of protein synthesis in cell-free extracts. Two striking features common to these inductions are the lack of biological specificity of the dsRNA inducer, and the exceptional sensitivity to low levels of dsRNA. The broad objective of the proposed research program is to determine the biochemical mechanism(s) by which dsRNA induces these effects with particular emphasis on the regulation of protein synthesis. The similarity of inhibitory properties of dsRNA in cell-free extracts and in cultured cells suggests that common mechanisms may be involved in the effects of dsRNA in these various biological systems. The sensitivity of rabbit reticulocyte lysates to inhibitors of protein synthesis provides a convenient assay system to monitor the presence of protein synthesis inhibitors in dsRNA-treated cells and extracts. Parameters to be examined include (a) the role of protein chain initiation factors in the mechanism(s) of inhibition; (b) the alteration of protein kinase activity profiles with emphasis on the modification of initiation factors by phosphorylation; (c) the characterization of dsRNA binding sites; and (d) the isolation and purification of dsRNA-induced inhibitors.